1. Technical Field
The present invention relates to an organic semiconductor thin film, a charge transport film, a method for producing the charge transport film, and a light-emitting element and photoelectric conversion element that use the charge transport film. In particular, the invention relates to a charge transport film which is useful for forming an organic photo-electronics element having plural functional thin films (layers), a method for producing the same, and a light-emitting element and photoelectric conversion element that are equipped with such a charge transport film.
2. Background Art
Organic electronics elements are elements which perform electric action by using organic substances, and have been expected to demonstrate strong points such as energy saving, low costs, and flexibility. Organic electronic elements have attracted attention as technologies replacing conventional inorganic semiconductors which mainly use silicone. In recent years, since lamination layer type organic electroluminescence elements prepared by using a vacuum deposition method have been on the market, studies for development of organic EL (electroluminescence) displays have become more popular, and now, organic EL displays are being put to practical use.
In such lamination layer type organic electroluminescence elements, plural organic layers (for example, a light-emitting layer, a hole injection layer, a hole transport layer, an electron transport layer, and the like) are integrated and disposed between an anode and a cathode. Formation of these organic layers is usually carried out through vacuum deposition of organic compounds having a relatively low molecular weight. However, vacuum deposition methods have a lot of problems with respect to production suitability such that large-scale facilities are needed, uniform thin films without defects are difficult to obtain, a long time is needed for forming plural organic layers in accordance with a deposition method, or the like. Similar problems exist, not only in the case of light-emitting elements, but also in the case of organic thin-film solar batteries containing plural organic charge transport films.
Therefore, technologies that construct the organic layers, not by a vapor phase method, but by a wet method such as coating or printing, are required from the viewpoint of improvement in productivity. However, in the case of providing plural layers in a laminated form by a wet method, mutual dissolution between the layers should be suppressed, and diffusion of effective components to adjacent layers at the interface of the layers should be suppressed. In order to solve the above problems, various means have been studied. For example, use of organic layers which form a crosslink structure by applying heat or radiation (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 7-114987; Daisuke Kumaki, Kengo Hirose, Nobuaki Koike, Akira Kuriyama, and Shizuo Tokitou, Proceedings of The First Regular Meeting of the Organic EL Panel Discussion, page 27 (2005); and H. Yan, P. Lee, N. R. Armstrong, A. Graham, G. A. Evmenenko, P. Dutta, and T. J. Marks, J. Am. Chem. Soc., vol. 127, pages 3172 to 4183 (2005)), and formation of a three-layer structure in which compounds having a large difference in solubility from each other are used in combination (see, for example, Y. Goto, T. Hayashida, and M. Noto, IDW'04 Proceedings of The 11th International Display Workshop, pages 1343 to 1346 (2004)) are known. However, all of the above means have problems in that element performance is insufficient, as well as in that usable raw materials are limited, stability of the raw material itself is insufficient, or the like. In order to solve these problems, use of an epoxy-based or oxetane-based crosslinkable polymer has been proposed (see, for example, JP-A No. 2007-302886).
Also in a case in which an organic cured layer utilizing a crosslink structure is prepared, these are problems in that organic low molecular weight components may diffuse in the cured layer, which may cause crystallization, and element performance may be deteriorated. Alternatively, even if crystallization is not caused, organic molecules which have been aligned, at first, in a molecular configuration suitable to charge transport may diffuse and move, whereby charge transporting capacity may be deteriorated.
Specifically, for example, organic electroluminescence elements (hereinafter, also properly referred to as organic EL elements) equipped with a hole transport layer including a hole transport material having a high hole transporting capacity, such as aromatic tertiary amines typified by N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (hereinafter, referred to as “TPD”), exhibit excellent initial light emission brightness and excellent initial light emission efficiency. However, the above organic EL elements have problems in that, after a very short time from the beginning of use, light emission efficiency is deteriorated and light emission brightness is deteriorated. One of the reasons for this phenomenon is thought to be as follows. Namely, since all of the above hole transport materials are low molecular weight materials, the melting point, glass transition temperature, crystallization temperature, and the like of the hole transport materials are low (for example, the glass transition temperature (Tg) of TPD is 63° C.), and thermal characteristics thereof are insufficient, and as a result, the hole transport material itself may be deteriorated due to the Joule heat which is generated when electric current flows to the element, or formation of exciplex between the hole transport material and a light-emitting material may easily occur.
Moreover, in organic EL elements, for the purpose of raising the carrier injection efficiency, the interface between organic layers and the interface between an organic layer and an electrode layer are finished to be as smooth as possible, and each of the above organic layers is made amorphous. However, it is thought that, when a hole transport material having a low molecular weight is used, since the crystallization temperature thereof is low, molecular aggregation occurs easily, whereby the carrier injection efficiency may be deteriorated or the light emission efficiency of the element may be deteriorated.
Therefore, it is preferable to fix the molecular configuration in order to prevent aging-induced changes in performance of the charge transport film, during preparation, during storage, or during driving of the organic electronics element using the charge transport film.
For fixing the molecular configuration, a usual method is to prevent the transfer of molecules by constructing a network structure of a polymer through adding a polymerizable monomer or a polymer having a crosslinkable group, and a photopolymerization initiator, and then performing photoirradiation to allow polymerization and crosslinking. However, with regard to the charge transport film in which high purity is required for the raw materials to be used, the existence of an initiator or its decomposition products is not preferable. The reason for this is as follows. Namely, when an initiator or its decomposition products exist, these may act as charge traps to remarkably deteriorate the charge transporting capacity and stability thereof over time. Therefore, in the case of using an initiator in an electroluminescence element, the initiator may cause lowering of brightness or deterioration in durability. Further, in the case of using an initiator in a photoelectric conversion element, the initiator may cause lowering of conversion efficiency or deterioration in durability. Accordingly, a method for performing polymerization fixation without using an initiator has been required.
As a polymerization method that does not use an initiator, plasma-initiated polymerization is known (see, for example, Y. Osada, M. Shen, and A. T. Bell, J. Polymer Sci., Polymer Letters Ed., vol. 16, pages 669 to 675 (1978)). However, since plasma irradiation is performed in vacuo or at a high temperature, batch processing is mainly performed, and since large-scale facilities are needed, it is not practical from the viewpoint of productivity. Moreover, since materials are exposed to a high temperature or electric discharge, there is a problem in that decomposition and deterioration of the surface of organic materials cannot be avoided. In 1987, it became possible to generate plasma at a relatively low temperature and under atmospheric pressure by performing intermittent discharge in a rare gas (see, for example, S. Kanazawa, M. Kogoma, T. Moriwaki, and S. Okazaki, Proceedings of Japan Symposium on Plasma Chemistry, vol. 3, page 1839 (1987)), and a technique of applying the atmospheric pressure plasma to a surface treatment of a base has been disclosed (see, for example, JP-A No. 2008-60115). However, even under a low temperature and atmospheric pressure, it was a concern that the influence upon the coating film at the time of direct irradiation of an organic layer with plasma was significant, and there have been no application of the technique to a coating film containing an electron transporting agent or a compound exhibiting liquid crystallinity was practically problematic.